1. Field of the Invention
The present invention relates to a compact large-magnification-factor zoom lens having a wider angle of view.
2. Description of the Related Art
Recently, design techniques and manufacturing techniques for optical equipments are advanced and downsize of zoom lenses and increase in magnification factors have become possible. Accordingly, a wide variety of large magnification factor zoom lenses have been proposed (for example, Japanese Patent Laid-Open Publication Nos. 2000-89117, 2002-236255, 2003-241097, 2005-331697, and H11-174327).
Zoom lenses described in the above Patent documents include a first through a fourth lens groups having refractive powers of positive, negative, positive, and positive, respectively, in an order arranged from an object side, thereby achieving an increased magnification factor.
Particularly, the zoom lens described in Japanese Patent Laid-Open Publication No. 2005-331697 has such a feature that, when the zoom lens is used in a digital single-lens reflex camera including an image sensor in an APS-C size installed therein, the zoom lens can ensure a back focus equivalent to that typically possessed by a conventional single-lens reflex camera for a 35 millimeters (mm) film.
Further, the zoom lens described in the Patent document 5 is configured to have six lens groups including a first lens group having a positive refractive power and a second lens group having a negative refractive power in an order arranged from an object side, thereby achieving an increased magnification factor.
Unlike conventional film single-lens reflex cameras, in digital single-lens reflex cameras, attachment of foreign matters onto an imaging device such as a charge-coupled device (CCD) leads to considerable deterioration of a photographed image. To prevent attachment of foreign matters onto imaging devices by allowing the decreased number of times of lens exchange, zoom lenses of large magnification factors are developed. However, such zoom lenses have photographing angles of view of about 75 degrees and F-numbers of about 3 to 4, and zoom lenses having magnification ratios exceeding 13 have not been proposed.
Meanwhile, increased magnification ratios of zoom lenses lead to increased displacements of lens groups as well as increased variations of aberrations, thereby causing a problem that aberration corrections are made difficult over the entire magnification ranges, respectively. To solve this problem, methods are conventionally adopted to conduct aberration correction by decreasing refractive powers of lens groups of a zoom lens, or to conduct aberration correction by forming a surface(s) of one(s) of lenses constituting a zoom lens as an aspherical surface(s). However, decreased refractive powers of the lens groups result in increased displacement of the lens groups upon magnification change and thus in a complicated cam mechanism that supports the zoom lens, thereby newly causing a problem of an increased dimension of the zoom lens in a radically outward direction thereof.
In the zoom lenses described in Japanese Patent Laid-Open Publication Nos. 2000-89117, 2002-236255, and 2003-241097, aspherical surfaces are introduced and third lens groups are each constituted of three lenses, thereby conducting aberration corrections without increasing the number of lenses. According to such configurations, aberration corrections can be conducted without substantial problems, insofar as magnification ratios are limited to about 7 to 10. However, the configurations each suffer from failure of correction of spherical aberrations caused in a first lens group and a second lens group, respectively, at a telephoto end, when the magnification ratio is set to be about 13.
Furthermore, the zoom lens described in Japanese Patent Laid-Open Publication No. 2005-331697 has a magnification ratio of about 7, so that the magnification of the third lens group is rarely made to be about −1 due to magnification change even by decreasing a refractive power of the third lens group. However, if the magnification ratio is increased to about 13 while the configuration is unchanged, a focal length where a magnification is made to be −1 is included, and aberration correction at the third lens group is brought to a limit, thereby causing a problem of deteriorated optical performance of the zoom lens as a whole.
Also, the zoom lens described in the Japanese Patent Laid-Open Publication No. H11-174327 has the same problem as those zoom lenses described in the other Patent documents.
In any case, it is impossible to sufficiently correct aberrations caused at the first and the second lens groups at a telephoto end, when magnification ratios are made to be 13 or more in the configurations of the zoom lenses described in the above Patent documents. This rather leads to increased sensitivities of the third lens groups to aberrations, respectively, thereby causing a problem that even a slight manufacturing error is not allowed.